Transposons (transposable elements) are genes that can move ("jump") from one DNA molecule to another in a cell, or from one location to another on the same DNA molecule. They can facilitate the transfer of genes, such as antibiotic-resistance genes, from the chromosome of a bacterium to a plasmid. They also can contribute to genetic diversity by causing mutations.

The simplest type of transposon is an insertion sequence (IS). It is a sequence of DNA that encodes an enzyme called transposase, which enables the IS to move. The transposase gene is flanked on either side by fifteen to twenty-five base pairs, arranged as "inverted repeats." A composite transposon is composed of any gene sandwiched between two IS sequences; this entire unit will move.